Radio signal receiving system



May 12, 1942. w. R. KOCH RADIO SIGNAL RECEIVING SYSTEM Filed June 29, 1940 INN, NNWAKK Patented May 12, 1942 i RADIO SIGNAL RECEIVING SYSTEM Winfield R. Koch, Haddoneld, N. J., assgnor to Radio Corporation of America, a corporation of Delaware Application June 29, 1949, serial No. 343,139 Y 12 Claims.

This invention relates to a radio signal receiving system having a variable-frequency band width within which it is-responsive to received signals. More particularly, this invention relates to a frequency-modulated signal receiving system, the band width of which is responsive to variations in the modulation depth or frequency swing.

As is known, the band width in frequency-modulation signal receivers, such as the pass band of the intermediate-frequency amplifier, for example, is preferably variable automatically in response to variations in the frequency swing or modulation of a received signal, in order at all times to pass thevdesired Vsignal while excluding undesired signals and noise potentials which would otherwise pass through to the output of` f the receiving system if the band width were maintained sufficiently wide at all times lto accept signals with full modulation.

In frequency-modulation signalV transmission and receiving systems, the percentage modulation is normally relatively low, being of the order of to 50 percent. Assuming 100 percent modulation of a carrier or mid-frequency signal of 40 mc. to be i 100 kc, deviation or frequency swing, the normal modulation may be of the order of i 20 to 50 kc. deviation or frequency swing. Thus, the band width required to pass such signals with full modulation and without distortion; is from 39.9 to 40.1 mc. lConsiderable noise and undesired signal' voltages may be conveyed through the signal receiving systemif `the wide band response is maintained for conditions of relatively low percentage modulation.

It is, therefore, desirable to vary the selectivity or the effective selectivity of frequency-modulation and similar wide band types of receiving systems, to prevent the transmission of noise `and undesired signals therethrough, and it is an object of the present invention to provide an improved selectivity control system for wide band radio signal receiving apparatus which is auto-V matically responsive to variations in signal strength, frequency swing, or modulationto pass the desired signal while excluding undesired signal and noise potentials. l A

It is a further object of the present invention to provide a band-width control system for a relatively wide-band signal receiving system which lacks certain inherent disadvantages of known variable band-width signaling systems. For ex-` ample, it is known that in order to change the band width of the frequency response of a bandpass filter requires control of the circuit'tuning,

reactance and lthe terminating rethat this may be accompanied by in' signal gain lthrough the the coupling sistance, and an undesirable change system.

It istherefore, a still'further objectof the present invention to provide means whereby the effective selectivity of a radio signal receiving system may be controlled automatically `in response to variations in signal strength or fre-V quency swing without appreciable change in gain through the receiving channel under band-'width control, A v` In carrying into effect the tion as referred to, it has been found'that the effective band width of the receiving system may be variedrby passing the received signal through two successive filter networks substantially equal in design'and frequency response characteristicsk and providing sharp signal attenuation'at a frequency adjacent to but outside the useful side bands on one side of the carrier or mean'frequency and varying the position of the carrier or the mean frequency with respect tothe attenuation frequency of the filter network first on one side of the carrier and then on the other,

successively,V as the signal is passed through theA signal receiving system, preferably inlthe inter` mediate-frequency amplifier portion thereof.

It is, therefore, a further object of the invention to provide a band-widthqcontrol system automatically responsive to the strength or lfre-v quency swing of received-signals, for securing a change in the eifectiveselectivity of at least Va portion of said system without appreciable change in gain therethrough by maintaining the signal carrier or mean frequency in variably spaced relation to the cut-olf point of a dual lter network, thereby removing interference signals lying outside the side bands of the narrowest signal band desired.

In accordance with the 1nvention, this is ac-A complished by-V changingthe frequencies of two heterodyneoscillators, connected with converters in the signal channel in an opposite sense one with'respect to the other, thereby to shiftthe carrier or mid-frequency with respect to the cutoff points of two lter networks through which the signal is passed. With thissystem;v ithas f been found that the gain through the system may be substantially unaifected by *changein effective selectivity.

-The invention will, however, be further understood from the following description, when conysidered in connection with the accompanying,

objects of the invenmeans are provided.

Figures 3, 4 and 5 are graphs representing certain operating characteristics of the receiving system of Figs 1 kand 2.

Referring to Fig. 1, a superheterodyne type of radio signal receiving system is represented, by way of example, including a first detector or heterodyne converter 6 coupled to a signal source such as an antenna 'I and to the usual heterodyne oscillator indicated at 8, to provide' intermediatefrequency signals for intermediate-frequency circuits represented at 9 and I). These circuits areV separated to include Vcircuits hereinafter described and may: contain the usual amplification anda frequency discriminator networkat l!) preceding the second detector indicated at Il,` from which audio frequencysignals are derived for the audio frequency amplifier indicated at I2 and the output device or loudspeaker I3 coupled thereto.

The receiving systemreferredto may beconsiderecl as representingV any suitable signal receivingjsystem, preferably ofthe superheterodyne type, providing an intermediate-frequency signal conveying system with or Without amplification, in which band-Width control Since the invention is more particularly adapted for` use in connection-"with frequencymodulation: receiving systems,l it may be assumed, by way of example and for purposes of illustration; that the embodiment represented inFig. 1, and as subsequently referred to in Fig. 2, is a frequency-modulation receiving system.

The intermediate-frequency*circuits 9 and I0 in the present system are connected by; a band-width control system signalkconveying channelrk having an effective band-widthvariation in response tothe variationv of the frequency of two oscillators l5 and I5, the former beingV a relatively high frequency oscillator and thelatter being a relatively low frequencyA oscillator,vwith respect tothe i. f. carrier or mid-frequency. f

Each. of the oscillators is provided with frequency control. means indicated at Il and I8,V

respectively, inturn connected through a control circuit I9 with the second detector II or other suitable source'v ofV variable. frequency control potential responsive to variations in the percentage modulation orsignal strength.

The signal channel between the i. f. circuits 9 and I lcomprises two conversion sections, one associated with each control oscillator. In the first section, coupled to thek first i. f..circuit 9, an intermediate-frequency band-pass filter 2B is coupled'between a first converter 2l and a second" converter 22 associated with the first orhigh frequency oscillator l5. Between the two sections a 'connecting intermediate-frequency Ycircuit- 23 is provided'. 1

The second' section, coupled* tothe circuit 23, is similar to the first and includes a second intermediate-frequency band-pass filter 24 coupled between a third converter 25 and a fourth converter ,ZSgiassociated with the lowfrequency or v second oscillator I6. The fourth converter is VId whichV forms ,ay

coupled to the detector Il through the discriminator network or intermediate-frequency circuit I0.

The high frequency oscillator l5 supplies oscillations to the rst and second converters on either side of the first i. f. band-pass filter 20. Likewise, the low frequency oscillator l 6 supplies signals to the third and fourth converters on either side of the second i. f. band-pass filter 24. Both i. f. band-pass filters preferably have the same wide band-pass characteristic and sharp cut-off or side band attenuation characteristic` for one side of the carrier and side bands. Variation in the oscillator frequencies causes the carrier'ormea-n.frequency of the signal in each section to move toward and away from the filter attenuation or 'cut-olf point in response to variations ,inp signal strength.

AThe first converter serves to invert or transpose the 'side bands, resulting in attenuation of the upper side band or the noise signals adjacent thereto; with-.lowpercentage modulation, vin passing. through thei.,f..filter 20, and the second converterY serves,V to restore the side bands to theirlfcrmerposition relative to thercarrier or mean frequency and torestorethe signal to the chosen intermediate frequency. The third con.- verter is provided to change the intermediatefrequency signal, without inversion of the side bands, tothe same frequency as that applied to the first intermediate-frequencyfilter, in .order that the other side band'V or the undesired signals adjacent thereto may similarly beattenuated in the i. f. filter 24. The fourth converter is for thepurpose of restoring the i. f. signal to its former frequency for the i.;f. circuit I0, which cuit elements as in Fig. 1, for comparison with the general arrangement shown in Fig. l.

In Fig. 2the intermediate-frequency circuits 9A are provided Vbvo-an intermediate frequency couplingy transformer of the tuned primary-secondary t-ype. YThis is coupled to the signal input grid 30 of the first converter which includes a converterA vtube A3l'. Oscillations may be applied-tothe converter in any suitable manner. However, in the present-example, the tube is of the combined oscillator-detector type having an oscillation anode electrode 32and an oscillator gridf33 coupled through a tunable oscillator cir-v cuit comprising a feedback winding 34 in circuit with the anode electrodeand a tuned grid circuit coupled thereto and connected to the v grid33, the usual-grid coupling capacitor 36 and grid resistor 3l being provided in connection therewith.

As indicatedk in Fig. *1, the -second converter likewiseV receives oscillations Yfrom the same source as thefirst converter and includes a' tube 40 whichKVA likewise may be of the oscillator-de-- tector type, provided with an oscillator anode electrode 4l andanoscillator rgrid electrode 42 connected in parallel with the corresponding electrodes in the rst converter. lSignals from 1 In Fig.- 2`the same reference nu the first converter are conveyed through the band-pass lter 28 to the signal input grid 43 of the second converter. The second converter is followed by the i. f. circuits 23 with preferably are the same as the i. f. circuits 9 and tuned to the same frequency. f i

The second sectionl of theband-width control system is preferably the same as the first section, in order that it may have the same frequency characteristics, thereby to impart to the signal conveyed therethrough the same attenuation characteristics on opposite sides of the carrier or mid-frequency, although the second section may have other characteristics, if desired.v

In the present system the third converter, indicated by the tube 45, andthe fourth converter, indicated by the tube 46, are, coupled in the same manner as the first and secondconverters, on opposite sides of the second band-pass filter 24, and are provided with similar oscillator circuits comprising a feedback winding 41 and a tuned oscillator circuit 48, the oscillator elements being connected in parallel with the circuit elements in order to impart the same frequency to both the third and the fourth converters.l Y

The i. f. circuit I0, coupled to the output of the fourth converter 46, is in the form of a frequency discriminating network for the detector which is a double diode rectifier 49 having a differential output impedance 50-5I for lderiving and rectifying an amplitude-modulated signal from the frequency-modulated signal and applying the same to the remainder of the receiving system through an amplifier 52 coupled through a lead 53 to the detector output impedance. The output connection for the amplifier 52 is indicated by the output lead 54 through which audio frequency signals `are conveyed to further amplifier means and an output device or loudspeaker, as indicated in Fig. 1.

The low frequency and high frequency oscillators are each controlled in frequency by suitable means such as a reactance tube having an anode circuit 6l coupledto the tuned circuit 35 for the high frequency oscillator and having a control grid 62 connected with a variable bias control circuit 63 through a filter network 64-65-66 for'applying to the grid 62 a variable bias potential, whereby the reactance of the tube B8 and the frequency of the oscillator as established by the'tuned circuit 35 `is varied Within predetermined limits.

For reactance control of both oscillators jointly,

the grid 62 is coupled through a capacitor 61 and a series high resistance element 68:to the grid circuit 69 of both oscillators, thereby being-coupled back to the anode through the grid capacitor 35. The grid 62 is further coupled to the cathode 69 of the reactance tube through a capacitor 18. v 4

Likewise, the third and fourthconverter. low frequency oscillator circuit 48 is controlled by a reactance tube 1I, the anode circuit 12 of which is connected with the circuit 48 .While the .control grid 13 is coupled to the common oscillator grid circuit 14 through a capacitor 15.v The grid 13 is further connected with the control circuit 63 through a resistor element16. Coupling between the anode circuit 12 and the grid 13 is provided` through the oscillator grid capacitor 11,. y

The frequency control means for lboth-*oscil-Y lators is responsiveto variation in a singlebias potential means, receiving variations in the con-` trol potential through the control lead 63. The

controlling potential is derived from thesignal channel of the system following the detector II, and 'in the present example, in connection with the audio frequency output circuit 54 from the first audio frequency Vamplifier stage 52.

lIn this connection, a rectifier is coupled to the circuit 54 through "a suitable cuplin'gfca` pacitor 8l, and the output' impedance of the rectifier, indicated at 8.2, is connected between ground 83 and the control 1ead`63, thereby to provide a variable positive bias potential on the grids 62 and 13 of the reactance tubes 60 and 1I, respec- 68, while, in the'low frequency oscillator control system, the series capacitor 15 may have a low reactance comparedwith the shunt resistance of element V16. f .Y Y' .f' With this arrangement, as the positive bias through the lead 63-increaseswith` increased frequency swing or signal "strength, the 'high frequency oscillator circuit 35 is increased in frequency, while the low frequency oscillator 48 is decreased in frequency, this being in directions to cause a decrease inthe selectivity of the bandwidth control system,that is, a-relatively wider frequency response or band width.

Y The-operationof the system may be better understood by considering,.for example, that a 10 `rnc-i. f. signal is appliedY to the first i. f. circuit 9 from the heterodyne converter. The rst -converter serves to transpose or invert the signal, so that the upper and lower side bands'are transposed in the signal which is passed through the first i. f. filter 20.

The high frequency oscillator operates'at a fre-A 'Y quency above the i. f. frequency and, in the present example, may be assumed to operate 'at a normal frequency of 16 mc.,.thereby providing a- 6' mc. signal through the filter 2|) with thel side bands inverted.V Ther inversion is' repeated through the second converter 40, so that'the signal with the side bands restored to normal-relation to the mid-frequency isagain at 10.mc; in

passing through the second i; f. circuit 23. r

Referring particularly to Figs. 3 and`4, assuming an i. f. signal at 10 mc., indicated at 85, with side bands 86 and 81, and that the side bands are those provided in response to normalor relatively low modulation such as 20 kc. on either side of the mid-frequency 85, interfering signals or noise signals A and B adjacent lto the side bands on either side thereof may be-passed through with Ynals A and B, Aand the filter 20 is'provided with a sharp attenuation characteristic -on one side of the signal, such as the low frequency side as indicated at 88, to cut off the interfering signalB,`

which is on the high side of the signal band normally. i

' signals adjacent thereto tion with the l0 mc. signal at 23, a 6 mc. signal Both i. f. filters 2U and 24, therefore, are arranged to provide substantially identical frequency and Yattenuation characteristics, and in the present example'to attenuate signals on the low frequency side of the'mid-frequency, while the response on the opposite side of the carrier or mid-frequency is relatively broad, as indicated by the characteristic at 90. As indicated in Fig.

4, this broad characteristic would include one of the interference signals A which, however, is cut-off in the second i. f. filter, as indicated in Fig. 5, assumingV normal or low modulation or frequency swing.

As the-signal strength increases or the frequency swing becomes wider, the control potential for the frequency control means of -both oscillator circuits 35 and 48 is increased to cause thehigh frequency oscillator to increase in frequency, while the low frequency oscillator is decreased in frequency, thereby shifting the midfrequency 85', as indicated in Figs. 3, 4 and 5, away from the attenuation points of the filter characteristics and permitting the wider frequency modulation to pass through the signal channel. This causes the effective band width of the signal channel to vary with signal strength or frequency swing and, since the variation is effected by means of oscillator frequency variation, the gain through the system ls not changed thereby, as in certain band width control systems heretofore known.

By changing the frequency of the two control oscillatorswith variations in signal strength or frequency swing, the signal may be placed close to the cut-off points of the filters, and noise and undesired signals, and any portion of the high frequency side' band may be attenuated in the first i; f. filter 20, while any portion of the low frequency side bands and the noise and undesired may be attenuated in the second i. f. filter 24|.y l

Thus, it is desirable to provide both filters 20 and 24 with the-same attenuation characteristics and ofthe order of those indicated in Figs. 4 and 5, so that the same frequency characteristic and attenuation is imparted to the signal on both sides ofthe mean frequency or carrier.

`The fourth converter serves to restore the intermediate frequency to the normal value as in the input circuits for conversion to amplitudemodulated signals in the frequency discriminator network I0 and detector 49.

Since the oscillators must be operated to provide a frequency shift in opposite directions the same number `of kilocycles, the reactance tubes must be connected so that one 4gives a positive reactance and the other a negative reactance, as

indicated in Figa in` order that both may be" controlled by the same control voltage from the circuit 63 While the invention has been described in*- connection with a preferred embodiment thereof for a frequency modulation signal receiving sys- F Y tem, it is obvious that it may be applied to any signal receiving system adapted 'for band width control in 'an'intermediate portion of the signal channel. y I' I claim as my invention: Y

1. Ina frequency modulated signal receiving system, means for varying the effective selectivity of said system with substantially constant gain, comprising Atwo band pass filter networks each having a predetermined frequency response and attenuation characteristic providing relatively sharp attenuation on one side of the pass band .of the network; means for passing a received signal through one of said filter networks with the `side lbands inverted, and means for shifting the mid-frequency of a'received signal with respect to the attenuation side'of the pass band in each lter network in accordance with variations in the output signal voltage ofsaid system.

2."In a frequencymodulated signalreceiving system, means for varying the effective intermediate frequency selectivity of said system with substantially constant gain, comprising two intermediate frequency band pass filter networks each having substantially thev same frequency response and attenuation characteristic providing relatively sharp attentuation on one `side of the pass band of' the network, means for passing a received signal through one of saidr filter networks with the side bands inverted and through both filter networks at the same mean frequency, and'means for shifting the mid-frequency of a received signalV with respect to the attenuation side of the pass bandin eachV filter network in accordanceY withl variations in the output signal voltage of said system.

3. In a frequency modulated signal receiving system, means for varying the effective intermediate frequency selectivity of said system with substantially constant gain, comprising two intermediate frequency band pass filter networks each having apredetermined frequency response and attenuation characteristic providingY relatively sharp attenuation on one side'of the pass band of the network, means forpassing a received signal through one of said filter networks with the'side bands inverted, and means for shifting the mid-frequency of areceived signal with respect to the attenuation side of the pass band in each Vfilter network in accordance with variations in the frequency swing of a received signal. y

4. In a radio signal receiving system, means for varying the'effective selectivityof said system with substantiallyconstant gain, comprising an intermediate frequency amplifying channel, twol band pass filter networks in saidchannelk each having a predetermined frequency response and attenuation characteristic providing relatively sharp attenuation on'one side of the pass band of the ne'twork,a converter connected with one lter network to apply an inverted side band signal thereto'at a predetermined frequency, a variable frequency oscillator coupled to said converter andY tuned above the intermediate frequency, a second converter forrestoring the signal with normal side bands, a third converter connected with the filter network to apply the restored side band signal theretoat a predetermined frequency, a second variable frequency oscillator coupled to said third converter'and tuned belowv the intermediate frequency, and means responsive vto variations inthe frequency swing of a received signal ,for jointly varying the frequency of saidoscillators to varythe signal relative to the cutoff frequency of said filter networks therebyA to vary the effective selectivity of said. system. f l

5. In a radio signal receiving system, means for varying thev effective selectivity of's'aid system with substantially constant gain, comprising an intermediate frequency amplifying channel, two band pass filter networks in said channel each having a predetermined frequency response and attenuation characteristic providing relatively sharp attenuation on one side of the pass band of the network, a converter connected with one lter network to apply an inverted side band signal thereto at a predetermined frequency, a variable frequency oscillator coupled to said converter and tuned above the intermediate frequency, a second converter for restoring the signal with normal side bands, a third converter connected with the filter network to apply the restored side band signal thereto at a predetermined frequency, a second variable frequency oscillator coupled to said third converter and tuned below the intermediate frequency, a frequency discriminator network for receiving the signal output through said filter networks, frequency control means for said oscillators for varying the frequency thereof in opposite directions one with respect to the other, a single control circuit for said frequency control means, and means for deriving a control potential from said discriminator network for said control circuit.

6. In a frequency modulated signal receiving system, band width control means comprising a dual filter network and means for maintaining the signal mean frequency in variably spaced relation to a cut-off frequency of said filter network, means for inverting the signal preceding one portion of said filter network, and means responsive to variations in frequency swing of a received signal for controlling said second named means whereby the effective selectivity of said system is variable between predetermined limits.

7. In a radio signal-conveying system, the combination of a pair of filter circuits, means including a variable frequency oscillator connected with the first filter circuit for applying an inverted signal thereto, means for reinverting said signal, means including a second variable frequency oscillator connected with the second filter circuit for applying thereto said reinverted signal, means in said filter circuits for imparting thereto attenuation characteristics of the same order, and frequency control means for varying the frequencies of said oscillators in an opposite sense one with respect to the other, thereby to vary the effective band width of said signalconveying channel through said filter circuits.

8. In a radio signal-conveying system, the combination of a pair of filter circuits, means including a variable frequency oscillator connected with the first lter circuit for applying an inverted signal thereto, means for reinverting said signal, means including a second variable frequency oscillator connected with the second filter circuit for applying thereto said reinverted signal, means in said filter circuits for imparting thereto attenuation characteristics of the same order, and frequency control means responsive to variations in amplitude of the audio frequency output of said system for varying the frequencies of said oscillators in an opposite sense one with respect to the other, thereby to vary the effective band width of said signal-conveying 'channel through said filter circuits.

9; In a radio signal-conveying system, the combination of a pair 'of VAfilter circuits, converter means connected with the first filter circuit Vfor applying an' inverted signal thereto, a second converter means for reinverting said signal, a third converter means connected with the second filter circuit forY applying theretov said reinverted signal. at the same frequency as said vinverted signal, means in said filter circuits for imparting thereto attenuation and frequency response Y characteristics of the same order and band width,

an oscillator coupled to said first and second converters and operating at a normal frequency above the signal frequency, a second oscillator coupled to said third converter and operating at a normal frequency below the signal frequency, and frequency control means for varying the frequencies of said oscillators in an opposite sense one with respect to the other, thereby to vary the effective band width of said signal conveying channel through said filter circuits.

l0. In a radio signal receiving system, the cornbination of two filter networks having predetermined signal attenuation characteristics, means for passing a received signal through said filter networks successively including variable frequency oscillator and converter means connected with the input and output ends of one of said filter networks for inverting the side bands of a received signal preceding said filter network and restoring said side bands to a normal position relative to the carrier or mid-frequency following said filter network in the signal channel, means for converting the signal to the frequency of the second filter network including a variable frequency oscillator tunable below the signal frequency in the filter network, frequency control means connected with said oscillators to vary the frequency thereof in an opposite sense one with respect to the other, and means responsive to variations in the signal strength of a received signal for applying a controlling potential to said frequency control means, thereby to yeffectively shift a received signal in frequency Withrespect to the cut-off frequency of each of said lter networks, thereby to vary the effective selectivity of the receiving system automatically in response to variations in signal strength with substantially constant gain.

11. In a frequency modulated signal receiving system, the vcombination with an intermediate frequency portion of said system, of a pair of variable frequency oscillators one having an operating frequency above and the other having an operating frequency below the intermediate frequency of said system, a pair of frequency converters connected with one of said oscillators, a filter network interposed in circuit between said converters, a second pair of converters, a second filter network interposed between said converters, said filter networks having substantially equal frequencyresponse and band pass characteristics and a relatively sharp cut-oli for signal attenuation on one side of a mean pass band frequency, and means for deriving a controlling potential from said system which varies in accordance with variations in frequency swing of a received signal, and means responsive to variations in said potential for varying the frequency of said oscillators in an opposite sense one with respect to the other, thereby to vary the effective selectivity of said receiving system through said filter networks.

12. In a frequency modulated signal *receiving system, band width control meanscomprising successive signal conveying portions having p redetermined cui-9h frequencies, a frequency disrminwr ansldetetpr network Coupled to.. said bandwidth control means, means for maintaining the signal mean fijequency in variably spaced reanni@ Phasen Qui-0a frequencies of Saman width control means, means `for presentingrthe signal successively to different portionsgof said 1o limits.

WINFIELD R. KOCH. 

